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Improvement of divertor triple probe system and its measurements under full graphite wall on EAST
Fusion Engineering and Design 84 (2009) 57–63
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improvement of divertor triple probe system and its measurements under full graphite wall on EAST Tingfeng Ming ∗ , Wei Zhang, Jiafeng Chang, Jun Wang, Guosheng Xu, Siye Ding, Ning Yan, Xiang Gao, Houyang Guo Institute of Plasma Physics, Chinese Academy of Sciences, PO Box 1126, Hefei 230031, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 28 September 2008 Accepted 27 October 2008 Available online 18 December 2008 Keyword: Divertor Langmuir probe
a b s t r a c t Full graphite wall of experimental advanced superconducting tokamak (EAST) has been developed in the spring of 2008. A new divertor triple probe diagnostics system (DTPDs) is built for EAST during this upgrade. The tip shape and connected structure of the probe are optimized for variational magnetic field directions and DTPDs maintenance. The experiment has been carried out with a full graphite wall for EAST, and near double-null diverted plasma is achieved successfully. The evolutions of electron temperature, density, particle flux and power densities along the divertor targets have been obtained with DTPDs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Divertor configuration has been adopted in many magnetic fusion machines in present, and the physical study of divertor is one of the crucial tasks in many instruments [1]. Langmuir probe arrays have been used as a common diagnostic tool, for it has a relatively simple and can give information for edge plasma in tokamak, such as density, temperature and potential simultaneity with high temporal and spatial resolutions [2–5]. EAST is a fully superconducting tokamak, which has been built at the Institute of Plasma Physics, Chinese Academy of Sciences in 2006, aiming to achieve steady-state operation with electron temperature Te larger than 10 keV, and electron density ne larger than 1020 m−3 [6]. The engineering design for EAST requires that the maximum power flux to the target plates during the first phase of operation should be below 1 MW/m2 to ensure long pulse, longer than 100 s, operation. Since there are so many interesting divertor physics to investigate to ensure EAST steady-state operation, essential diagnostics for divertor is required. Langmuir probe is one of the good choices, as demonstrated in the first campaign on EAST in January 2007 [7,8]. However, these probe tips are tilted, which make probe sensitive to the direction of the magnetic field, consequently, probes could not work normally if the direction of magnetic field changed or even reversed; and for its old structure, probe fixed on the plates cannot be renovated if damaged. Thus, probe tip shape and its connected structure have been optimized.
∗ Corresponding author. Tel.: +86 551 5593342; fax: +86 551 5591310. E-mail address: [email protected] (T. Ming). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.10.005
In this article, new optimized DTPDs for the EAST machine is reported. The design specifications and overall arrangements of EAST DTPDs are described in Section 2, and the experimental results are presented in Section 3. Finally, a summary is given in Section 4.
Fig. 1. An overview of divertor triple probes embedded poloidally in EAST (UI and UO are the abbreviations of upper inner and upper outer, respectively).
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T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
Fig. 2. A top view of the toroidal distribution of DTPDs.
Table 1 An overview of probe number installed on different regions. Divertor
Inner target
Outer target
Dome
Lower (37 × 3 = 111) Upper (37 × 3 = 111)
15 × 3 15 × 3
20 × 3 20 × 3
2×3 2×3
Fig. 4. Evolutions of the measured main plasma and upper outer divertor plasma parameters: (a) plasma current, (b) line-averaged density, and (c)–(g) ion saturation current of upper outer divertor probes embedded in F window.
2. Experimental setup The construction of the new DTPDs, which consists of 222 Langmuir probes, is accomplished during the upgrade for EAST in spring of 2008. Probes are embedded in the lower and upper divertor regions, including inboard, outboard, and dome plates, because EAST has a flexible poloidal field control system to accommodate both single-null and connected double-null divertor
configuration, and adopted ITER-like vertical target configuration with tightly fitted side baffles and a central dome in the private region to physically separate the inner and outer divertors. These probes are assembled into 74 group triple probes and the tips are spaced toroidally to ensure the plasma uniformity [9], to measure plasma basic parameters, such as electron temperature, electron density, particle flux and heat flux in diver-
Fig. 3. Cross-sections of Langmuir probe assembly: (a) the old design, and (b) improved design.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
59
Fig. 5. Evolutions of the measured plasma parameters at the upper inboard (probe UI11–UI15) and inner dome (probe UI16): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
60
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
Fig. 6. Evolutions of the measured plasma parameters at the upper outboard (probe UO02–UO18 in F window): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
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Fig. 7. Evolutions of the measured plasma parameters at the lower inboard (probe LI09–LI14): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
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T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
tor region. An overview of their numbers is summarized in Table 1. Take the upper divertor for instance (the arrangement are the same for lower divertor, with replacement for U to L in the order names), the total number for triple probe embedded on the inner target, inner dome, outer dome and the outer target are 15, 1, 1 and 20, with a relevant order named as UI01–UI15, UI16, UO21, and UO01–UO20 respectively, see as Fig. 1. The spatial resolution of these divertor probes is 1–1.5 cm. To achieve this, four windows, i.e. D, E, F and G windows, have been impropriated for triple probe installation. A top view of the distribution is given in Fig. 2. For its old connected structure (which is wedged, see as Fig. 3(a)), Langmuir probe fixed on the divertor plates, could not be renovated if tips were destroyed, during the campaign in January 2007. Considering the ineluctable damage of tips working under long pulse and high power operation in future, the connection between tip and copper screw have been optimized. The new connected structure is designed to be screw thread, by opening a tap hole with female thread from the bottom of graphite tip, see as Fig. 3(b). With this structure, the maintenance for DTPDs becomes easier, by renovating damaged tips from the front in the vacuum chamber. Since the discharge parameters on EAST are low in the first phase, the power handling problems are not serious. In order to reduce the effect from the angle with respect to the field line and ensure the ion saturation current saturating enough, the probe tip is machined into a dome (round) shape, which is 8 mm in diameter, with a partial 6.08 mm spherical radius end, in this upgrade for EAST DTPDs. These probe tips are made of graphite (EDM-3). All of the insulators are machined from machinable glass ceramic, and copper components are made from chromium-zirconium-copper, while SS are 316L stainless steel.
The effective collection area of the probe is 8.2 mm2 using the strong field model. One of the triple probes provides the floating potential while the other two probes are applied with a 200 V bias. When the probe is biased sufficiently negatively, all electrons are repelled and all that remains is the ion saturation current being independent of voltage. So the electron temperature (in eV, and 1 eV = 11,600 K) of target has a simple expression [10],
A1 A2
,
(1)
where V+ is a positively biased potential, Vf is a floating potential, and A1 , A2 are the collection areas of the two tips. Given A1 = A2 , Eq. (1) can be written as [9], Tt = (V+ − Vf )/ln 2.
(2)
For sheath-limited plasma, the particle flux collected by a probe is [10] i = n∞
1 1/2 s
Cs,
(3)
where, Cs = (2Tt /mi )1/2 is the ion-acoustic speed at the target, s = 0.854 and mi is the ion mass, e and Aeff are the electron charge and effective collection area of the tip, respectively. According to the relation 2nt = n∞ [1], the particle density at the target nt can be written as nt =
Iis , 2 eAeff Cs
P = nt Cs(Tt + εpot )
(5)
where εpot is the potential energy of each incident ion (εpot ≈ 16 eV), and is the sheath transmission factor. Assuming there is no secondary electron emission and Ti = Te , the sheath transmission factor = 6.8, for pure deuterium plasma. Near double-null diverted plasmas with full graphite wall are achieved successfully and verified by the fitting code EFIT (GA) after the upgrade for EAST. DTPDs has been used to measure parameters during elongation and strong-shaped plasma discharges. The data from all the probes at a rate of 5 kHz can be acquired simultaneously. By using such data of positively biased potential, floating potential, ion saturation current and Eqs. (2)–(5), and taking the sin () law [9] for particle and power fluxes into account, real-time edge plasma parameters such as electron temperature, electron density, particle flux and power flux at the target plate can be obtained. For a 2 T toroidal magnetic field condition, the results of shot 8332 presented here is in Ohmically heated with a plasma current of approximately 250 kA, and a line-integral density of approximately 1.7 × 1019 m−2 , with deuterium as the working gas. The pulse duration is about 4.7 s with a current plateau time 2.6 s. Evolutions of plasma current, main plasma density and ion saturation currents collected by probes installed on upper outboard of F window, are shown in Fig. 4. The discharge starts with a limiter configuration, and a divertor configuration shapes at 2.1 s. The measured results of the edge plasma parameters by DTPDs from different divertor plates, i.e. lower target (inboard) and upper targets (both inboard and outboard), are given from Figs. 5–7, where (a)–(d) are evolutions of electron temperature, electron density, particle flux and power flux at the plates. 4. Summary
3. Experimental results from DTPDs
Tt = (V+ − Vf )/ln 1 +
The power flux is given by [1]
(4)
New DTPDs for EAST has been developed in the upgrade for EAST. The designs of probe tip shape and the connected structure have been optimized. The DTPDs has been used successfully in the measurement for edge plasma parameters; nevertheless, the measured results from the lower and partial upper outboard have been unsatisfactory owing to the installation of the heat sink. Little proposal on divertor physics has been actualized, for the primary task is the test of isoflux control technology and engineering commissioning under graphite wall condition for EAST in the last campaign. Further studies, such as gas puffing, asymmetries between the inner and outer targets, different regimes (i.e. sheath-limited, conductionlimited and plasma detachment) of EAST divertor, will be studied with higher discharge density and input power in future, and the EAST DTPDs will be available on the measurement for edge plasma parameters. Acknowledgements The authors would like to thank the EAST team for joint experimental study. This work was supported by National Nature Science Foundation of China under Grant No. 10605028 and No. 10728510. References [1] C.S. Pitcher, P.C. Stangeby, Plasma Phys. Control. Fusion 39 (1997) 779. [2] J.G. Bak, S. G. Lee, and the KSTAR Project Team, Rev. Sci. Instrum. 74 (2003) 1578. [3] N. Tsois, C. Dorn, G. Kyriakakis, M. Markoulaki, M. Pflug, G. Schramm, et al., J. Nucl. Mater. 266–269 (1999) 1230.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63 [4] J.G. Watkins, R.A. Moyer, J.W. Cuthbertson, D.A. Buchenauer, T.N. Carlstrom, D.N. Hill, et al., J. Nucl. Mater. 241–243 (1997) 645. [5] P.J. Harbour, D.D.R. Summers, S. Clement, J.P. Coad, L. De Kock, J. Ehrenberg, et al., J. Nucl. Mater. 162–164 (1989) 236. [6] Y. X. Wan, HT-7 Team, HT-7U Team, Nucl. Fusion 40 (2000) 1057. [7] X. Gao, and the EAST team, Phys. Rev. A 372 (2008) 2286.
63
[8] J. Wang, X. Gao, G.S. Xu, W. Zhang, J.F. Chang, H.Q. Liu, et al., Phys. Scripta 78 (2008) 035501. [9] G.F. Matthews, Plasma Phys. Control. Fusion 36 (1994) 1595. [10] L.H. Hutchinson, Principles of Plasma Diagnostics, 2nd ed., Cambridge University Press, Cambridge, 2002.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improvement of divertor triple probe system and its measurements under full graphite wall on EAST Tingfeng Ming ∗ , Wei Zhang, Jiafeng Chang, Jun Wang, Guosheng Xu, Siye Ding, Ning Yan, Xiang Gao, Houyang Guo Institute of Plasma Physics, Chinese Academy of Sciences, PO Box 1126, Hefei 230031, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 28 September 2008 Accepted 27 October 2008 Available online 18 December 2008 Keyword: Divertor Langmuir probe
a b s t r a c t Full graphite wall of experimental advanced superconducting tokamak (EAST) has been developed in the spring of 2008. A new divertor triple probe diagnostics system (DTPDs) is built for EAST during this upgrade. The tip shape and connected structure of the probe are optimized for variational magnetic field directions and DTPDs maintenance. The experiment has been carried out with a full graphite wall for EAST, and near double-null diverted plasma is achieved successfully. The evolutions of electron temperature, density, particle flux and power densities along the divertor targets have been obtained with DTPDs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Divertor configuration has been adopted in many magnetic fusion machines in present, and the physical study of divertor is one of the crucial tasks in many instruments [1]. Langmuir probe arrays have been used as a common diagnostic tool, for it has a relatively simple and can give information for edge plasma in tokamak, such as density, temperature and potential simultaneity with high temporal and spatial resolutions [2–5]. EAST is a fully superconducting tokamak, which has been built at the Institute of Plasma Physics, Chinese Academy of Sciences in 2006, aiming to achieve steady-state operation with electron temperature Te larger than 10 keV, and electron density ne larger than 1020 m−3 [6]. The engineering design for EAST requires that the maximum power flux to the target plates during the first phase of operation should be below 1 MW/m2 to ensure long pulse, longer than 100 s, operation. Since there are so many interesting divertor physics to investigate to ensure EAST steady-state operation, essential diagnostics for divertor is required. Langmuir probe is one of the good choices, as demonstrated in the first campaign on EAST in January 2007 [7,8]. However, these probe tips are tilted, which make probe sensitive to the direction of the magnetic field, consequently, probes could not work normally if the direction of magnetic field changed or even reversed; and for its old structure, probe fixed on the plates cannot be renovated if damaged. Thus, probe tip shape and its connected structure have been optimized.
∗ Corresponding author. Tel.: +86 551 5593342; fax: +86 551 5591310. E-mail address: [email protected] (T. Ming). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.10.005
In this article, new optimized DTPDs for the EAST machine is reported. The design specifications and overall arrangements of EAST DTPDs are described in Section 2, and the experimental results are presented in Section 3. Finally, a summary is given in Section 4.
Fig. 1. An overview of divertor triple probes embedded poloidally in EAST (UI and UO are the abbreviations of upper inner and upper outer, respectively).
58
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
Fig. 2. A top view of the toroidal distribution of DTPDs.
Table 1 An overview of probe number installed on different regions. Divertor
Inner target
Outer target
Dome
Lower (37 × 3 = 111) Upper (37 × 3 = 111)
15 × 3 15 × 3
20 × 3 20 × 3
2×3 2×3
Fig. 4. Evolutions of the measured main plasma and upper outer divertor plasma parameters: (a) plasma current, (b) line-averaged density, and (c)–(g) ion saturation current of upper outer divertor probes embedded in F window.
2. Experimental setup The construction of the new DTPDs, which consists of 222 Langmuir probes, is accomplished during the upgrade for EAST in spring of 2008. Probes are embedded in the lower and upper divertor regions, including inboard, outboard, and dome plates, because EAST has a flexible poloidal field control system to accommodate both single-null and connected double-null divertor
configuration, and adopted ITER-like vertical target configuration with tightly fitted side baffles and a central dome in the private region to physically separate the inner and outer divertors. These probes are assembled into 74 group triple probes and the tips are spaced toroidally to ensure the plasma uniformity [9], to measure plasma basic parameters, such as electron temperature, electron density, particle flux and heat flux in diver-
Fig. 3. Cross-sections of Langmuir probe assembly: (a) the old design, and (b) improved design.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
59
Fig. 5. Evolutions of the measured plasma parameters at the upper inboard (probe UI11–UI15) and inner dome (probe UI16): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
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T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
Fig. 6. Evolutions of the measured plasma parameters at the upper outboard (probe UO02–UO18 in F window): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
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Fig. 7. Evolutions of the measured plasma parameters at the lower inboard (probe LI09–LI14): (a) plasma current and electron temperature, (b) plasma current and electron density, (c) plasma current and particle flux, and (d) plasma current and power flux.
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T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63
tor region. An overview of their numbers is summarized in Table 1. Take the upper divertor for instance (the arrangement are the same for lower divertor, with replacement for U to L in the order names), the total number for triple probe embedded on the inner target, inner dome, outer dome and the outer target are 15, 1, 1 and 20, with a relevant order named as UI01–UI15, UI16, UO21, and UO01–UO20 respectively, see as Fig. 1. The spatial resolution of these divertor probes is 1–1.5 cm. To achieve this, four windows, i.e. D, E, F and G windows, have been impropriated for triple probe installation. A top view of the distribution is given in Fig. 2. For its old connected structure (which is wedged, see as Fig. 3(a)), Langmuir probe fixed on the divertor plates, could not be renovated if tips were destroyed, during the campaign in January 2007. Considering the ineluctable damage of tips working under long pulse and high power operation in future, the connection between tip and copper screw have been optimized. The new connected structure is designed to be screw thread, by opening a tap hole with female thread from the bottom of graphite tip, see as Fig. 3(b). With this structure, the maintenance for DTPDs becomes easier, by renovating damaged tips from the front in the vacuum chamber. Since the discharge parameters on EAST are low in the first phase, the power handling problems are not serious. In order to reduce the effect from the angle with respect to the field line and ensure the ion saturation current saturating enough, the probe tip is machined into a dome (round) shape, which is 8 mm in diameter, with a partial 6.08 mm spherical radius end, in this upgrade for EAST DTPDs. These probe tips are made of graphite (EDM-3). All of the insulators are machined from machinable glass ceramic, and copper components are made from chromium-zirconium-copper, while SS are 316L stainless steel.
The effective collection area of the probe is 8.2 mm2 using the strong field model. One of the triple probes provides the floating potential while the other two probes are applied with a 200 V bias. When the probe is biased sufficiently negatively, all electrons are repelled and all that remains is the ion saturation current being independent of voltage. So the electron temperature (in eV, and 1 eV = 11,600 K) of target has a simple expression [10],
A1 A2
,
(1)
where V+ is a positively biased potential, Vf is a floating potential, and A1 , A2 are the collection areas of the two tips. Given A1 = A2 , Eq. (1) can be written as [9], Tt = (V+ − Vf )/ln 2.
(2)
For sheath-limited plasma, the particle flux collected by a probe is [10] i = n∞
1 1/2 s
Cs,
(3)
where, Cs = (2Tt /mi )1/2 is the ion-acoustic speed at the target, s = 0.854 and mi is the ion mass, e and Aeff are the electron charge and effective collection area of the tip, respectively. According to the relation 2nt = n∞ [1], the particle density at the target nt can be written as nt =
Iis , 2 eAeff Cs
P = nt Cs(Tt + εpot )
(5)
where εpot is the potential energy of each incident ion (εpot ≈ 16 eV), and is the sheath transmission factor. Assuming there is no secondary electron emission and Ti = Te , the sheath transmission factor = 6.8, for pure deuterium plasma. Near double-null diverted plasmas with full graphite wall are achieved successfully and verified by the fitting code EFIT (GA) after the upgrade for EAST. DTPDs has been used to measure parameters during elongation and strong-shaped plasma discharges. The data from all the probes at a rate of 5 kHz can be acquired simultaneously. By using such data of positively biased potential, floating potential, ion saturation current and Eqs. (2)–(5), and taking the sin () law [9] for particle and power fluxes into account, real-time edge plasma parameters such as electron temperature, electron density, particle flux and power flux at the target plate can be obtained. For a 2 T toroidal magnetic field condition, the results of shot 8332 presented here is in Ohmically heated with a plasma current of approximately 250 kA, and a line-integral density of approximately 1.7 × 1019 m−2 , with deuterium as the working gas. The pulse duration is about 4.7 s with a current plateau time 2.6 s. Evolutions of plasma current, main plasma density and ion saturation currents collected by probes installed on upper outboard of F window, are shown in Fig. 4. The discharge starts with a limiter configuration, and a divertor configuration shapes at 2.1 s. The measured results of the edge plasma parameters by DTPDs from different divertor plates, i.e. lower target (inboard) and upper targets (both inboard and outboard), are given from Figs. 5–7, where (a)–(d) are evolutions of electron temperature, electron density, particle flux and power flux at the plates. 4. Summary
3. Experimental results from DTPDs
Tt = (V+ − Vf )/ln 1 +
The power flux is given by [1]
(4)
New DTPDs for EAST has been developed in the upgrade for EAST. The designs of probe tip shape and the connected structure have been optimized. The DTPDs has been used successfully in the measurement for edge plasma parameters; nevertheless, the measured results from the lower and partial upper outboard have been unsatisfactory owing to the installation of the heat sink. Little proposal on divertor physics has been actualized, for the primary task is the test of isoflux control technology and engineering commissioning under graphite wall condition for EAST in the last campaign. Further studies, such as gas puffing, asymmetries between the inner and outer targets, different regimes (i.e. sheath-limited, conductionlimited and plasma detachment) of EAST divertor, will be studied with higher discharge density and input power in future, and the EAST DTPDs will be available on the measurement for edge plasma parameters. Acknowledgements The authors would like to thank the EAST team for joint experimental study. This work was supported by National Nature Science Foundation of China under Grant No. 10605028 and No. 10728510. References [1] C.S. Pitcher, P.C. Stangeby, Plasma Phys. Control. Fusion 39 (1997) 779. [2] J.G. Bak, S. G. Lee, and the KSTAR Project Team, Rev. Sci. Instrum. 74 (2003) 1578. [3] N. Tsois, C. Dorn, G. Kyriakakis, M. Markoulaki, M. Pflug, G. Schramm, et al., J. Nucl. Mater. 266–269 (1999) 1230.
T. Ming et al. / Fusion Engineering and Design 84 (2009) 57–63 [4] J.G. Watkins, R.A. Moyer, J.W. Cuthbertson, D.A. Buchenauer, T.N. Carlstrom, D.N. Hill, et al., J. Nucl. Mater. 241–243 (1997) 645. [5] P.J. Harbour, D.D.R. Summers, S. Clement, J.P. Coad, L. De Kock, J. Ehrenberg, et al., J. Nucl. Mater. 162–164 (1989) 236. [6] Y. X. Wan, HT-7 Team, HT-7U Team, Nucl. Fusion 40 (2000) 1057. [7] X. Gao, and the EAST team, Phys. Rev. A 372 (2008) 2286.
63
[8] J. Wang, X. Gao, G.S. Xu, W. Zhang, J.F. Chang, H.Q. Liu, et al., Phys. Scripta 78 (2008) 035501. [9] G.F. Matthews, Plasma Phys. Control. Fusion 36 (1994) 1595. [10] L.H. Hutchinson, Principles of Plasma Diagnostics, 2nd ed., Cambridge University Press, Cambridge, 2002.